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Financial Instruments - Schedule Of Available-For-Sale Investments With Unrealized Losses (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value $ 1,996.3 $ 2,680.2
Unrealized losses less than 12 months (7.2) (13.4)
Unrealized losses 12 months or greater Fair Value 880.4 0.0
Unrealized losses 12 months or greater (8.2) 0.0
Total Fair Value 2,876.7 2,680.2
Total Unrealized Losses (15.4) (13.4)
Corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value 567.6 1,056.1
Unrealized losses less than 12 months (2.1) (5.0)
Unrealized losses 12 months or greater Fair Value 277.0 0.0
Unrealized losses 12 months or greater (2.3) 0.0
Total Fair Value 844.6 1,056.1
Total Unrealized Losses (4.4) (5.0)
U.S. treasuries    
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value 763.5 357.1
Unrealized losses less than 12 months (2.5) (2.0)
Unrealized losses 12 months or greater Fair Value 206.2 0.0
Unrealized losses 12 months or greater (2.2) 0.0
Total Fair Value 969.7 357.1
Total Unrealized Losses (4.7) (2.0)
U.S. government agencies    
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value 428.9 538.2
Unrealized losses less than 12 months (1.3) (2.9)
Unrealized losses 12 months or greater Fair Value 345.5 0.0
Unrealized losses 12 months or greater (3.1) 0.0
Total Fair Value 774.4 538.2
Total Unrealized Losses (4.4) (2.9)
Municipal securities    
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value 236.3 728.8
Unrealized losses less than 12 months (1.3) (3.5)
Unrealized losses 12 months or greater Fair Value 51.7 0.0
Unrealized losses 12 months or greater (0.6) 0.0
Total Fair Value 288.0 728.8
Total Unrealized Losses $ (1.9) $ (3.5)